Liquid/liquid extraction is a laboratory and industrial technique for separating a dissolved solute from a solution thereof which has been in use for many years. Such method comprises one or more steps in which a first liquid solution of a solute in a first, relatively non-volatile, solvent is vigorously mixed with a second, usually organic, solvent. The second solvent, while having a high affinity for the solute material, is both much more volatile than the first solvent and substantially immiscible therewith. Recovery of the solute is accomplished by separating the two liquids and then evaporating the second solvent to allow the recovery of the solute therefrom. Such method, while relatively simple to perform, suffers from the disadvantage that the most suitable volatile solvents are materials such as hexane, benzene, ether, acetone, methyl chloride, acetonitrile and chloroform. All of these materials present substantial flammable and/or toxic hazards so that proper handling and recovery constitutes an ever more stringent set of environmental and economic problems. In extreme cases, the costs of disposing of these materials can be anywhere from 5 to 10 times the initial cost of the organic solvent. Further, when such an approach is used, for example, in the testing of urine or blood samples for one or more drugs, the relatively long times required for such an extraction causes serious problems in many high-volume analytical laboratories. With all of this, it has been reported that, as recently as 1984, of the 400 million plus analytical samples prepared and tested in the nation's biological, clinical, pharmaceutical, toxicological, forensic, environmental, chemical, food and cosmetic laboratories, over 60 percent were still prepared by liquid/liquid extraction.
One emerging technology being used to overcome these problems is known as solid phase extraction. Using this method, the first solution containing solvent is passed through a cartridge containing a selected solid inorganic or organic sorbent phase to extract the solute from the solvent. One typical sorbent material for this purpose is an ion exchange resin which removes dissolved salts of calcium, iron and similar mineral material from water in many household and industrial water softener units. In many cases, the desired product is usually the "softened" water. In these units the resin is rejuvenated by periodically backflushing the resin bed with a salt solution and discarding the flush solution to a public sewer. In other cases, the extracted mineral material is of interest, and the backflush solution is retained in the system to recover the dissolved solute. While many large units based on this technique can be found in industry, there has not been a concomitant development of special, relatively small, automated units utilizing this approach to meet the needs of forensic, toxicology, clinical and other high volume analytical laboratories.